1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus for capturing, oxidizing, lowering the concentration and/or level of, and/or eliminating mercury present in any flue gas and/or combustion gas stream. In one embodiment, the method and/or apparatus of the present invention is applied to boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices that have connected thereto at least one type of flue gas, or combustion gas, scrubber device (i.e., a wet scrubber or a dry scrubber).
2. Description of the Related Art
In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers and waste to energy plants. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very generally low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not substantially condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals in order to meet strict mercury stack emission limits (e.g., MATS in the United States, the European Community's mercury emission limits and/or regulations, and/or any other countries' and/or organization's mercury emission limit guidelines and/or regulations). To complicate matters, mercury can exist in its oxidized (Hg2+) form, principally as mercuric halide (e.g., HgCl2, HgBr2, HgI2, etc.), or in its elemental (Hg0) form as vaporous metallic mercury. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and various other factors.
The search for industrially acceptable methods for the capture of mercury from industrial flue gases has included a significant effort to determine how much mercury can be removed by existing, conventional air pollution control equipment, such as wet or dry scrubbers.
Accordingly, tests have been performed on several commercial scale and pilot scale wet scrubbers. In addition to being applicable to dry scrubber situations, these tests have produced some expected and some surprising results. It was generally expected that the oxidized mercury would be easily captured and the elemental mercury would be difficult to capture. These expectations were based on the high solubility of mercuric halides in water and the very low solubility of elemental mercury in water. This expectation was generally fulfilled.
The surprising result concerned elemental mercury. Repeated tests during which the concentration of elemental mercury in the flue gas was measured revealed that more elemental mercury was leaving the wet scrubber than was entering.
While not wishing to be bound to any one theory, it is believed that various ions present in wet scrubber slurries and/or in the flue gas stream of wet and/or dry scrubbers cause reduction of a portion of any oxidized mercury present in a flue gas and/or combustion gas stream converting same back to elemental mercury (Hg0). This portion is then emitted out of the stack of, for example, a power plant as elemental mercury is much more difficult to capture in any one or more downstream emission control devices and/or downstream air quality control system (AQCS) devices.
Thus, there is a need in the art for a method that accomplishes both an acceptable level of mercury oxidation (e.g., “a high degree of mercury oxidation”) in a flue gas, or combustion gas, stream as well as a method that simultaneously accomplishes control of mercury emission from a scrubber, be it wet or dry.